In a pay-per-period computing model, an accurate timer/clock is necessary to ensure correct billing and allocation of services. The temptation may exist to tamper with the clock to obtain free services. Existing computer systems may not provide secure system clocks. For example, existing computer systems may be susceptible to user tampering of clock date and time values. This may be unacceptable in a billing model which relies on an accurate system clock to determine when a user has consumed an allocated amount of computer usage or when an expiration date has passed, requiring further deposit of funds for continued use.